How to understand Drag and Drop MIME types ?

Hello everybody, I am coding a simple window, with two QListWidget's inside. My aim was to be able to drag and drop, elements of the QListWidget from one list to another... I was then wondering about, how I should understand MIME types... I tried to read about it in the documentation, but I am sure, someone explanation could be really interesting for me to better understand the meaning...Thanks in advance.